Human lymphoblastoid interferon has been purified to a specific activity of 4 x 10 to the 8th power International Units/mg protein. The homogenity of this preparation has been suggested by the appearance of a single band on analytical SDS polyacrylimide gel electrophoresis, and by attainment of constant specific activity and amino acid composition after an additional purification step. A complete amino acid analysis of purified interferon analytical tryptic peptide maps and CNBr cleavage fragments of small amounts of high specific activity radiolabelled interferon have been obtained. Work is now in progress to determine the N-terminal amino acid sequence of lymphoblastoid interferon by LKB solid phase and Beckmann spinning cup sequenators.